Electricity is a primary power source used in residential and commercial applications. Measuring the amount of electricity consumed is an important task in determining how much to charge a user. In addition, historic trends of electrical consumption are an important tool in determining how much electricity needs to be produced at any given time.
Electricity is generally delivered over power lines as alternating current. Electricity can be supplied as single-phase, double-phase, or triple-phase. Each additional phase generally necessitates using an additional power line. Additional phases can also be supplied with additional lines.
Electricity is generally measured in terms of an amount of total power consumed. Total power consumption generally involves the integration of power used over a specified period of time. Power is the product of voltage (V) and current (I). Electricity meters generally measure total power consumed by measuring the product of the voltage and the current at a given instant in time and accumulating (e.g. integrating) the product over time.
One way to measure the current is by using a current transformer. A current transformer generally includes a series of coil loops that encircle a toroidal core. When loaded with a burden resistor, the coil loop produces a voltage proportional to the current running through the power line. Current transformers are generally preferred for industrial and commercial poly-phase metering due to the current transformer's inherent DC isolation from phase to phase.
A second way to measure current is by using a Rogowski coil. A Rogowski coil generally includes a toroidal coil placed around a conductor whose current is to be measured. A voltage is induced in the coil which is proportional to the rate of change of the current in the conductor. The output of the Rogowski coil is generally connected to an electrical integrator circuit in order to provide an output signal that is proportional to the current. Rogowski coils are generally less expensive than current transformers.
Another way to measure the current is by using a current shunt. A current shunt generally includes a relatively small resistor which is placed in series with the power line. The current flows through the current shunt creating a small voltage across the shunt. A voltage meter then measures the voltage drop to determine the amount of current flowing through the current shunt. Current shunts by themselves are not isolated from the power line and are therefore not preferred for multiphase power measurement as phase-phase difference voltages produced on the input pins of power metering device can easily exceed maximum levels allowed in standard semiconductor products. Current shunts are generally less expensive than current transformers and Rogowski coils, however, due to the isolation issues, current transformers are preferred for commercial applications.